Clandestine nuclear weapons are an immediate threat to every country and every city in the world. A rogue nation with a nuclear weapon, or a terrorist group that acquires radiological material, could deliver it to a victim city via commercial shipping at low cost and low risk. Nuclear weapons are difficult to detect when shielded. Advanced radiation detectors are necessary to reveal such weapons among backgrounds and benign clutter. An urgent priority of the United States, and indeed of all countries, is the development of radiation detectors that both detect and localize clandestine nuclear material.
A signature of all nuclear and radiological weapons is radiation, principally gamma rays (“gammas”) and neutrons. Gamma rays are detected when they interact with matter via photoelectric absorption in which the gamma ray is absorbed and a photoelectron is emitted, Compton scattering which generates a Compton electron and a scattered gamma ray, or electron-positron pair production. In each case, the energetic electron (or positron, treated as an electron herein) can be detected in a charged-particle detector such as a scintillator, semiconductor, or gaseous ionization detector. Neutrons are usually classified according to energy as fast, intermediate, and slow. A fast or high-energy neutron, as used herein, has 100 keV to several MeV of energy. Fast neutrons can be detected by neutron-proton elastic scattering in which the recoil proton passes through a detector such as a scintillator. Slow or low-energy neutrons (1 eV or less, also called thermal or epithermal) can be detected by a capture reaction in a neutron-capture nuclide, usually 10B or 6Li, causing emission of prompt ions such as alpha and triton particles which can be detected in a scintillator or other ionization detector. Intermediate-energy neutrons can be moderated or decelerated by multiple elastic scattering in a hydrogenous material such as HDPE (high-density polyethylene) or PMMA (polymethylmethacrylate), and then detected as slow neutrons.
A directional detector that determines the location of a radiation source would be extremely helpful in detecting and localizing clandestine threats. Even a few gamma rays or neutrons coming from the same location could trigger an alarm. What is needed, then, is a gamma ray or neutron detector system that indicates the direction of the source, preferably with enough sensitivity to detect a shielded source from a single data set acquired at a single orientation. Preferably such a detector system would be compact, fast, efficient, easy to build, easy to use, and low in cost.